550 5.1.10 RESOLVER.ADR.RecipientNotFound

Microsoft 365 could not find the recipient

Seen in Microsoft 365, Exchange Online, Outlook NDRs

Exchange Online looked up the recipient's address in its directory and found no mailbox, contact or group with that address.

Common causes

A typo in the address, or an old address saved in Outlook autocomplete.
The mailbox was deleted, unlicensed or renamed.
In a hybrid setup, a cloud-only mailbox has no matching object on-premises (or the reverse), so routing fails.
Spammers forged your address and the bounce is backscatter you can ignore.

Check the address with the recipient and remove stale autocomplete entries.
If it is your tenant, confirm the user exists, has a mail licence and owns that address as a proxy address.
In hybrid, create the matching remote mailbox (New-RemoteMailbox) and let directory sync run.
Make sure the domain's MX record points to Microsoft 365.